Static pressure stabilizer for local exhaust

ABSTRACT

An exhaust system is provided for a plurality of machines, each machine having an exhaust stream connected to a local branch of the system. The system includes a stabilizer connected to an exhaust line in parallel to the exhaust stream of a machine. The stabilizer controls an exhaust pressure in the exhaust stream by opening and closing in response to variations in the exhaust pressure; the stabilizer opens to admit gas flow into the exhaust line when the exhaust pressure is less than a predetermined value, and closes when the exhaust pressure is greater than that predetermined value. The stabilizer unit thus provides a constant static exhaust pressure. The stabilizer provides relief flow in the exhaust stream, causing the gas flow into the exhaust line to increase when an exhaust flow in the exhaust stream decreases.

FIELD OF THE INVENTION

This invention relates to building exhaust systems in integrated circuit manufacturing facilities (“IC fabs”). More particularly, this invention relates to control of local static pressure in such systems.

BACKGROUND OF THE INVENTION

A modern IC fab has numerous sophisticated machines for performing manufacturing processes on semiconductor materials. These machines are referred to in the industry as “production tools” or simply “tools.” In general, each production tool is connected to a building-wide exhaust system for removing air or spent process gases from the interior of the tool. A typical building exhaust system is shown schematically in FIG. 1. Multiple fans and/or scrubbers 1 are connected in a manifold 2 to provide controlled, stable exhaust at header point 3. From this point a distribution system branches out in different directions throughout the building, with branches 4, sub-branches 5, and local branches 6 connecting to each of the tools (not shown).

Optimum operation of the production tools requires that the exhaust be as stable as possible. Even though a stable exhaust is provided at the fan manifold/header level (so that the overall system is stable), there are often fluctuations in the exhaust where a local branch connects to a set of tools (so that a particular zone of the system is not stable). A given tool will typically not have a constant exhaust; for example, a tool exhaust flow may vary due to the process being performed. The adjacent tools in the tool set will then be subjected to variations in the exhaust static pressure. This has a negative effect on the performance of those tools. For example, as shown in FIG. 2, a variation in exhaust flow at tool 11 will change the local lateral static pressure in local branch 6, affecting the other tools in tool set 10. Process monitoring or safety requirements may force shutdown of a tool if the static pressure variation is too great (for example, a −20% or +40% departure from normal). Besides the loss of tool uptime, any degradation in performance of the production tools may be detrimental to the product yield. Accordingly, static pressure variations at each local tool exhaust (e.g. local exhaust 12 of tool 11) must be minimized.

One method of solving this problem is to provide a local barometric damper which opens at a set static pressure. However, a barometric damper does not hold a constant static pressure as the flow varies. Another approach is to bleed air into the exhaust system. This can reduce the overall variation in local static pressure, but increases the load on the system and therefore makes the system more expensive to operate.

There is a need for a device and method for maintaining a constant static pressure at the local exhaust of a tool, even when the exhaust flow fluctuates.

SUMMARY OF THE INVENTION

The present invention addresses the above-described need by providing a static pressure stabilizer unit which, when connected in parallel with the exhaust stream of a production tool, maintains a constant static exhaust pressure at that tool.

According to a first aspect of the invention, an exhaust system is provided for a plurality of machines, each machine having an exhaust stream connected to a local branch of the system. The system includes a stabilizer connected to an exhaust line in parallel to the exhaust stream of a machine. The stabilizer controls an exhaust pressure in the exhaust stream by opening and closing in response to variations in the exhaust pressure; the stabilizer opens to admit gas flow into the exhaust line when the exhaust pressure is less than a predetermined value, and closes when the exhaust pressure is greater than that predetermined value. The stabilizer unit thus provides a constant static exhaust pressure. The stabilizer provides relief flow in the exhaust stream, causing the gas flow into the exhaust line to increase when an exhaust flow in the exhaust stream decreases.

The gas flow may be a flow of ambient air.

According to another aspect of the invention, a static pressure stabilizer is provided for a local exhaust stream of a machine connected to an exhaust system. The static pressure stabilizer includes a first pipe, a second pipe and a float inside the second pipe. One end of the first pipe is connected to the exhaust system while the other end has a first opening therein to admit gas flow. The second pipe is set in the first opening and has a wall with a second opening therein. The float is capable of movement along an axis of the second pipe; in a first position of the float the first opening is closed, thereby preventing gas flow into the first pipe, and in a second position the second opening communicates with the first opening, thereby permitting gas flow into the first pipe. The gas flow may be a flow of ambient air. When the stabilizer is connected to the exhaust system in parallel to the local exhaust stream, the stabilizer is effective to maintain a constant static exhaust pressure for the machine. The weight of the float may be adjusted in accordance with a desired value of the static exhaust pressure.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 schematically illustrates a typical building exhaust system, showing a distribution of branches to which production tools are connected.

FIG. 2 shows a typical production tool set, in which an exhaust flow variation in one tool impacts the performance of adjacent tools.

FIG. 3 shows the tool set of FIG. 2 with a static pressure stabilizer installed, in accordance with an embodiment of the present invention.

FIG. 4A shows construction of a static pressure stabilizer according to an embodiment of the invention.

FIG. 4B illustrates operation of the static pressure stabilizer of FIG. 4A.

FIGS. 5A and 5B show how the static pressure stabilizer provides a constant static pressure according to the present invention.

FIGS. 6A and 6B show operation of a conventional barometric damper, in contrast to the operation of the static pressure stabilizer of FIGS. 5A and 5B respectively.

DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT OF THE INVENTION

FIG. 3 illustrates an embodiment of the invention, in which a static pressure stabilizer device 30 is installed at local tool 11 in the tool set 10 of FIG. 2. Stabilizer 30 is connected to an exhaust line 13 in parallel to the exhaust stream 12 of tool 11. Stabilizer 30 opens (admitting a flow of room air 100) and closes as the local static pressure at tool 11 changes, due to the process being run in tool 11 or possibly some other factor. The result is that static pressure variations in the local exhaust are minimized.

Details of stabilizer 30 are shown in FIG. 4A. An outer pipe 31, connected to the exhaust system, has a capped end 31 a with an opening in which is set an inner concentric pipe 32. One or more slots 32 a are provided in the wall of pipe 32. A cylindrical float 33 fits inside pipe 32 and is able to move up and down along the axis thereof. In its resting position float 33 closes off the bottom opening of pipe 32; this is the closed state of stabilizer 30. An annular cap 34 prevents float 33 from falling out of the opening. If the static pressure decreases due to a fluctuation in the exhaust flow, ambient pressure forces the float upwards (see FIG. 4B). Air flow 100 enters the stabilizer through the slots 32 a, providing a relief flow which compensates for the reduced exhaust flow from the tool. This has the effect of increasing the static pressure, which in turn increases the downward force on the float and causes the float to return to its resting position.

Stabilizer 30 has been constructed from PVC pipe material, with pipes 31 and 32 having diameters of 10 inches (25.4 cm) and 6 inches (15 cm) respectively. Float 33 may be made from any convenient chemically stable material, sized to move smoothly inside pipe 32. A typical weight for float 33 is 1 pound (0.454 kg). Float 33 also need not be a solid cylinder, provided that the same circular area is presented on the top and bottom ends.

The stabilizer may be calibrated by adjusting the weight of the float 33 in accordance with the desired static pressure. FIG. 5A illustrates the situation where the float is in its resting position, and the upward and downward forces on the float are balanced. If the float is just at the point of lifting up, the upward force due to room air pressure is balanced by the downward force of the static exhaust pressure combined with the weight of the float. The downward force is given by F₁=W+SP1*A, where W is the weight of the float, A the area of each end of the float, and SP1 is the static exhaust pressure. The upward force is given by RP*A, where RP is the room air pressure. Thus, when the stabilizer is about to open, SP1=RP−(W/A). Any decrease in the static pressure will cause the float to lift inside pipe 32 and permit air to flow into the exhaust stream (FIG. 5B). If the static pressure rises above SP1, the downward force on the float will exceed the upward force, and the float will move back down to its resting position. The stabilizer 30 therefore maintains the static pressure at a value of SP1. It is noteworthy that as float 33 moves up and down inside pipe 32, surface 33 a on the exposed end of float 33 maintains the same angle with respect to the opening in end cap 31 a (in this case, surface 33 a is parallel to the opening).

The operation of the stabilizer 30 may be contrasted with that of a conventional barometric damper 60 with a hinged opening flap 61, shown in FIGS. 6A and 6B. FIG. 6A illustrates a situation similar to FIG. 5A, where the upward and downward forces on the flap 61 are balanced, so that the damper is at the point of opening. The downward force is given by F2=W+SP2*A, where W and A are the weight and area of the flap respectively; the upward force is RP*A. The static pressure at this point is therefore SP2=RP−(W/A). The angle of the flap with respect to the opening is clearly not constant. If the flap opens by an angle □ and forces on the flap are balanced (FIG. 6B), force F₃ is balanced by the force due to room air pressure RP*A. However, force F₃ is now given by F₃=W*cos □+SP3*A. Thus SP3=RP−(W/A) cos □. Since SP3>SP2 whenever the damper is open, the barometric damper will attempt to maintain a new pressure greater than SP2. Furthermore, since SP3 depends on the opening angle □, this new pressure will fluctuate as the flap 61 moves (SP3=SP2 at □=0°; SP3=RP at □=90°). Accordingly, the barometric damper cannot assure a constant static pressure.

A static pressure stabilizer such as described above may advantageously be installed at each production tool. This assures that the building exhaust system is stable both generally and locally, thereby contributing to reliable and effective tool operation.

While the invention has been described in terms of specific embodiments, it is evident in view of the foregoing description that numerous alternatives, modifications and variations will be apparent to those skilled in the art. Accordingly, the invention is intended to encompass all such alternatives, modifications and variations which fall within the scope and spirit of the invention and the following claims. 

1. An exhaust system for providing exhaust to a plurality of machines, each machine having an exhaust stream connected to a local branch of the system, comprising: a stabilizer connected to an exhaust line in parallel to the exhaust stream of a machine, wherein said stabilizer controls an exhaust pressure in said exhaust stream by opening and closing in response to variations in said exhaust pressure, the stabilizer opening to admit gas flow into the exhaust line when said exhaust pressure is less than a predetermined value, and closing when said exhaust pressure is greater than said predetermined value.
 2. An exhaust system according to claim 1, wherein the gas flow is a flow of ambient air.
 3. A system according to claim 1, comprising a plurality of stabilizers, wherein each machine connected to the local branch has a stabilizer connected in parallel thereto.
 4. A system according to claim 1, wherein said exhaust pressure is characterized as a static exhaust pressure for the machine, and the stabilizer associated with the machine maintains a constant static exhaust pressure for that machine.
 5. A system according to claim 3, wherein said exhaust pressure is characterized as a static exhaust pressure for each machine, and the stabilizer associated with a given machine maintains a constant static exhaust pressure for that machine.
 6. A system according to claim 1, wherein said stabilizer causes the gas flow into the exhaust line to increase when an exhaust flow in the exhaust stream decreases.
 7. A static pressure stabilizer for a local exhaust stream of a machine connected to an exhaust system, the static pressure stabilizer comprising: a first pipe having a first end and a second end, the first end connected to the exhaust system and the second end having a first opening therein to admit gas flow; a second pipe set in the first opening and having a wall with a second opening therein; and a float inside said second pipe and capable of movement along an axis thereof, so that in a first position of said float the first opening is closed, thereby preventing gas flow into said first pipe, and in a second position of said float the second opening communicates with the first opening, thereby permitting gas flow into said first pipe.
 8. A static pressure stabilizer according to claim 7, wherein the gas flow is a flow of ambient air.
 9. A static pressure stabilizer according to claim 7, wherein said stabilizer is connected to the exhaust system in parallel to the local exhaust stream, so that said stabilizer is effective to maintain a constant static exhaust pressure for the machine.
 10. A static pressure stabilizer according to claim 9, wherein a weight of said float is adjusted in accordance with a desired value of the static exhaust pressure.
 11. A static pressure stabilizer according to claim 9, wherein the constant static exhaust pressure has a predetermined value, and said float is given a force in a direction from the first position toward the second position when the static exhaust pressure is less than said value.
 12. A static pressure stabilizer according to claim 9, wherein the constant static exhaust pressure has a predetermined value, and said float is given a force in a direction from the second position toward the first position when the static exhaust pressure is greater than said value.
 13. A static pressure stabilizer according to claim 7, wherein said float has an end surface, the first opening defines a plane, and an angle of the end surface with respect to said plane remains constant during movement of said float.
 14. A static pressure stabilizer according to claim 8, wherein flow of ambient air is permitted into the first pipe in response to a decrease in exhaust flow in the local exhaust stream. 